1. Field of the Invention
This invention relates to a boring machine for machining or boring a semi-cylindrical workpiece, such for example as a semi-cylindrical intermediate product obtained during a process of producing a half bearing.
2. Description of the Related Technology
For example, in a process of producing a half bearing, a sheet piece of a rectangular shape is bent into a semi-cylindrical shape, and then an inner surface of this sheet piece is subjected to boring, thereby obtaining the precision of a bearing surface. There is known a boring machine which is provided with workpiece carry-in and carry-out means in order to efficiently bore a large amount of workpieces. In the conventional boring machine, as schematically shown in FIG. 14(A), a workpiece W.sub.1 is fixed to a workpiece fixing portion S mounted on a bed, and a spindle T, having cutting tools P and Q attached thereto, is movable in an axial direction. The workpiece fixing portion S is mounted on a moving mechanism (not shown), and when the boring processing for the workpiece W.sub.1, fixed to the workpiece fixing portion S, is finished, the workpiece fixing portion S is retracted or moved radially outwardly of the spindle T (FIG. 14(B)), and in this retracted position, the removal of the workpiece W.sub.1 from the workpiece fixing portion S and the attachment of a next workpiece W.sub.2 to the workpiece fixing portion S are effected. After the workpiece W.sub.2 is thus fixed to the workpiece fixing portion S, the workpiece fixing portion S is again moved or advanced into a processing (boring) position.
Recently, it has been required to process the inner surface of half bearings with a high degree of precision on the order of microns.
However, the workpiece fixing portion S is moved between the retracted position and the processing position each time the workpiece is subjected to the boring processing, and therefore it has been difficult to set the workpiece fixing portion S relative to the spindle T with the precision on the order of microns since this setting is influenced directly by a positioning error caused by the moving mechanism, and therefore the precision of processing of the workpiece has been limited.
After the boring processing is finished, the workpiece fixing portion S is retracted, and the processed workpiece W1 is removed from the workpiece fixing portion S, and after the next (unprocessed) workpiece W.sub.2 is supplied, and is fixed to the workpiece fixing portion S, the step in which the workpiece fixing portion S is again advanced into the processing position is needed, and the reduction of the time required for this step has been limited.